The present invention relates generally to the field of ink jet printing, and more particularly to an ink jet printer in which a plurality of commercially available ink jet printing heads are arranged or packaged in a manner which permits them to print an image of larger height than can be produced from a single print head.
In recent years ink jet printers have achieved significant popularity in various fields, particularly in the areas of desk top printing, such as computer printers, and other forms of convenience printing devices where the charateristics of major importance are convenience of printing from a small, easily movable, device having reasonably good speed and clarity of printed image. Perhaps the most popular and well known of these printing applications is that of ink jet printers for use with desk top computers, particularly those used in the home, where relatively low cost is another factor that contributes to the popularity of ink jet printers. However, technology is constantly improving the desirable characteristics of ink jet prirters and rendering them adaptable to a greater variety of applications, as a result of which additional demands are placed upon the technology by applications for these printers not anticipated during early stages of development.
A brief review of the underlying principles of operation of ink jet printers will facilitate a better understanding of the problems of using an ink jet printer in one of the aforementioned new applications. A simple ink jet printer consists of a print head having a suitable reservoir for holding a supply of ink, and a nozzle plate having a row of extremely small diameter holes or nozzles through which the ink is expelled onto a piece of paper as the print head moves across the paper. There are suitable conduit means for providing communication between the ink reservoir and each of the nozzles, and a minute resistance heater is positioned in each conduit so that when heated momentarily, it volitilizes the liquid ink that is adjacent to the heater to create a small babble, which in turn generates sufficient pressure in the conduit to force a minute droplet of ink from the nozzle associated with that heater. The heaters in all of the conduits are energized from a suitable power source in a predetermined sequence under the control of suitable software with the result that the droplets of irk ejected from the nozzles form a desired image on the piece of peiper as the print head moves across the paper. It will be apparent, of course, that in normal operation of the ink jet printer, the rate of sequential energization of the heaters and consequent ejection of ink jets from the nozzles is extremely rapid, a factor which is primarly responsible for the relatively rapid printing raite of these printers.
Ink jet printers were originally conceived and subsequently developed primarily to reproduce text and graphic images to accommodate the requirements of what is now commonly referred to as desktop publishing. In this field, the typical mode of operation of an ink jet printer was to print a line of text during one pass of the print head past the piece of paper. To do this, the ink nozzles were arranged in a pair of rows, usually extending about one quarter of an inch, the nozzles of one row being longitudinally offset very slightly from the nozzles of the other row and being spaced sufficiently close together to form a substantially solid line if all of the nozzles expelled a droplet of ink simultaneously. This configuration is generally the standard form of monochrome ink jet printing head incorporated in present day commercially available printers.
A significant problem inherent in printers as just described is that the vertical height of the printed image is limited to the corresponding length of the row of nozzles on the printing face, which, as previously stated, is about one quarter inch. While approximately one quarter inch height is entirely satisfactory for ordinary printed text, there are many situations in which such images as text headings and graphic materials exceed this height. In current ink jet printing technology, the dimensional limitation problem is circumvented by printing a portion of an image during one pass of the print head across the paper, and then indexing the paper to change its longitudinal relationship with the print head, and then printing the rest of the image during a second pass of the print head across the paper. If the image is sufficiently tall that it cannot be printed in two passes of the print head, then a third or ever subsequent passes may be required to print the entire image. It should be noted parenthetically that although reference has been made to moving the print head across; the piece of paper, this is not always the case, since in some printers the print head is maintained stationary, and the piece of paper is mounted for lateral movement in the printing device and moves with respect to the print head. All that is required in operation is relative movement between the paper and the print head.
It will immediately be recognized that there are several problems of differing degrees of severity involved in printing an image, whether text or graphics, in more than one pass of either the paper or the print head to print the entire image. One is the increased complexity of the software program that must fragment an image into horizontal sections and cause the print head to print the entire image in a plurality of horizontal sections. Another is the increased mechanical and electrical complexity of the paper feed mechanism in printers where the paper is moved relative to the print head during printing by now having to index the paper one printing line at a time in a direction perpendicular to the direction of printing.
However, a major problem encountered in situations where a graphic image is too tall to be printed in one pass of the paper or the print head, as the case may be, is where, for one reason or another, it is physically impossible to achieve more than one pass of the paper or the print head, with the result that if the image cannot be printed in one pass, it simply cannot be printed. This is the case with the printing application to which the present invention is related, which is the field of mailing machines. As is fairly well known, a mailing machine consists of a feed deck and a postage meter which includes a printing device for printing a postage indicia on the upper right hand corner of an envelope being fed through the mailing machine by suitable feeding means. Typically, printing is accomplished by passing the envelope between a curved printing die carried by a rotating drum and a backup pressure roller, during which ink previously applied to the printing die by a suitable inking device is transferred to the envelope, after which it is ejected from the mailing machine. In another mode, the envelope is pressed against a previously inked flat die by a moving platen to transfer the ink from the die to the envelope, after which it is ejected from the mailing machine. It is apparent that with either type of die, the image of the postage indicia, and often an accompanying advertising slogan, can be made to any desired size, which is limited only by the physical dimensions of the dies for the postage indicia and advertising slogan, and the envelope size, with the result that the entire image can be printed in one pass through the mailing machine.
Thus, there is a need for a technique for printing large dimension images, whether of text or graphics or both, by means of an ink jet digital printing apparatus in which the full height of the image can be printed during only one pass of the print head over the paper, or the paper over the print head, as the case may be.